"U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning."

The State Department encouraged travelers to check the websites of the U.S. embassies and consulates and register in the Smart Traveler Enrollment Program.

The travel alert is the fourth issued for Europe during the past year. The most recent went out last November and expired in February.

Monday's alert expires Sept. 1.

A French army police officer patrols the Eiffel Tower in Paris on Oct. 4, 2010. Security increased after the U.S. State Department issued travel advisories warning of potential terror threats from al-Qaeda in tourist destinations and public transport. The State Department Photo by David Silpa/UPI

The U.S. State Department issued a travel alert for Europe on Monday, saying U.S. citizens should be aware of a continued threat of terrorist attacks throughout the continent.

In the alert, the State Department cited recent incidents in France, Russia, Sweden and the United Kingdom and said Islamic State and al Qaeda "have the ability to plan and execute terrorist attacks in Europe."

The State Department's previous travel alert for Europe, issued ahead of the winter holiday season, expired in February. A State Department official said Monday's alert was not prompted by a specific threat, but rather recognition of the continuing risk of attacks especially ahead of the summer holidays. The alert expires on Sept. 1.

Malls, government facilities, hotels, clubs, restaurants, places of worship, parks, airports and other locations are all possible targets for attacks, the State Department's alert said.

US State Department issues travel alert for Europe

The US State Department has issued a travel alert for its citizens going to Europe, citing the continued threat of terror attacks.

"While local governments continue counterterrorism operations, the Department nevertheless remains concerned about the potential for future terrorist attacks," the alert said. "U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning."

The alert mentioned recent attacks in France -- which heads to the polls next week to choose a new president -- Russia, Sweden and the UK, noting that extremists tend to focus on tourist hot spots.

A US official told CNN in February that the terror threat in Europe was as high as it's ever been.Recent estimates from Europe and US say as many as 1,900 foreign fighters have returned to Europe after stints in Iraq and Syria, the official said. Some remain committed to terror groups like ISIS, but others returned disenchanted.